Can you put a throw rug or runner in the area that he needs to walk across? Slipping on a smooth floor is a true concern for a dog, and can lead them to being scared or actually even hurt themselves.
It's possible she slipped once & it startled her enough to cause the ongoing caution. Very often adolescent dogs go through a secondary fear imprint phase & can be even more effected by a single event that might even normally happen.That is what I am considering, it is behavior that just developed around 3 weeks ago..